Framework modification of porous materials has considerable potential to enable tuning of material properties to increase performance in a variety of applications such as separations, catalysis, and chemical sensors. In particular, improving material performance for separations is highly desirable to reduce the overall process energy requirements. Typically, the two material properties in porous materials that determine separation performance are pore size (kinetic separation) and adsorption selectivity (thermodynamic effect). From this perspective, a nanoporous class of metal-organic frameworks (MOFs) called zeolitic imidazolate frameworks (ZIFs) are an attractive option.
MOFs are a promising class of nanoporous materials for use in separations and catalysis, among many other applications. ZIFs, a subclass of these materials, have many advantageous properties including good thermal and chemical stability, high microporosity, and high surface area. The pore sizes of these materials (0.2-2 nm) allow selective sieving and recognition of molecules. Recent studies have also shown that ZIFs exhibit a “gate-opening” phenomenon: as they interact with adsorbing molecules, they undergo structural changes during adsorption, thereby allowing more adsorbate molecules into the framework. Because the organic linker components in the framework rotate to allow the above phenomena, the nature of the organic linker has significant implications on the selection and behavior of appropriate ZIF materials for specific applications. For instance, ZIF-8 has a crystallographic pore aperture of 0.34 nm as determined by X-ray diffraction; however, there is increasing evidence that the as-made material separates gases considerably larger than its pore aperture (e.g., C3H6/C3H8) more efficiently than gases closer to its crystallographically determined pore size (CO2/CH4).
In general, it is possible to tune the properties of MOFs for specific applications using methods such as chemical or structural modifications. One approach for chemically modifying a MOF is to use a linker that has a pendant functional group for post-synthesis modification. For example, ZIF-90, an aldehyde-containing ZIF, can be modified using NaBH4 as a reducing agent to generate alcohol groups. Another approach to modification is to use organic ligands that can change the structural characteristics of the material. MIL-53 exhibits a flexible framework, but modification of the terephthalic acid linker to include an amino functional group improves the separation performance for CO2. Another recent approach to modification is the use of a triazolate linker in which a C—H moiety of the imidazole is replaced by a nitrogen atom, thereby allowing crystallization of a hybrid material that does not disturb the crystal structure of the original material. However, in the case of using mixed linkers, determining appropriate ligand combinations a priori is not always straightforward. It has been shown that the use of ligands with bulky substituents produces new ZIF frameworks with enhanced CO2 adsorption properties by preventing crystallization of ZIF topologies with smaller unit cells and network cages; however, this discovery came from using high-throughput synthesis techniques. Similarly, the pore size of a MOF can be tuned by increasing the length of bridging organic linkers. A series of mixed-ligand Zn-based MOFs were transformed from a nonporous material to one with relatively high surface area and porosity by increasing the length of bridging dicarboxylic or bipyridyl linkers.
Another way to modify surface properties is by postsynthetic exchange (PSE) of the organic linkers or metal centers by heating the MOF material in a solvent containing a different linker or metal ion that exchanges into the material while maintaining the crystal structure. Recently, the linker of ZIF-71 (4,5-dichloroimidazole) was successfully subjected to PSE with a linker that is not otherwise found in ZIF structures (4-bromoimidazole). ZIF-8 has also been subjected to PSE, replacing the framework linkers (2-methylimidazole) with imidazole. This produced a material with 85% substituted linkers while maintaining the ZIF-8 crystal structure.
The different ZIF topologies can possess a variety of pore sizes and surface properties. ZIFs have been studied for CO2 adsorption and membrane-based separations by both experiments and computations. Although these materials normally have high CO2 capacity, the adsorption selectivity for typical gas pairs of interest (e.g., CO2/CH4) tends to be low and comparable with commercially available adsorbent materials such as BPL carbon. Practically, increasing the adsorption selectivity would greatly increase the potential for commercialization. Currently, very few ZIF materials (e.g., ZIF-78) have shown significant CO2/CH4 and CO2/N2 adsorption selectivities of 10 and 50, respectively, or more. Some large pore MOF structures exhibit higher CO2 affinity and selectivity for these gas pairs; however, these materials typically have open metal centers that are susceptible to performance degradation from steam exposure and poison from trace contaminants, which adversely affect CO2 capacity. Conversely, ZIFs have relatively high thermal and chemical stability that permits modification of the surface properties and have the added benefit of small pore apertures that are promising for kinetic gas separations, which further improves separation performance.